Wobbly numbers Jellyfish might be able to shut down a nuclear-powered aircraft carrier, but they are not taking over the world's oceans and turning them into slime, say scientists.

In the last decade, gelatinous blobs have reportedly clogged up water treatment and power plants, overrun fish farms, and hampered fishermen around the world. They even forced the nuclear-powered USS Ronald Reagan out of the Port of Brisbane in 2006.

But claims they will dominate our oceans are not supported by data, says Winthrop Professor Carlos Duarte director of the Oceans Institute at the University of Western Australia.

"There's no support as yet to make any assertion that jellyfish are rising globally," says Duarte.

Duarte is one of the co-founders of a group of international scientists involved in a three-year research project, known as the Jellyfish Data Initiative (JEDI), to examine claims that jellyfish numbers around the world are increasing.

An overview of their project is published in the latest issue of BioScience.

The group have assembled more than 500,000 records of published and unpublished data, some of which goes back to the 1790s, to examine the long-term patterns of jellyfish blooms.

While analysis of the data continues, so far it suggests that jellyfish blooms rise and fall in 20-year boom and bust cycles, says Duarte.

"The past period of rise was in the mid-90s to about 2005, and that's when all the papers claiming there was a global rise came out," he says.

"So in fact there was a little bit of support for that because there was more a general rise than the previous decade, but this is not unprecedented."

"If we looked at these with a longer time perspective we see that similar events have happened in the past."

According to Durant, one of the reasons why these boom and bust cycles have gone unnoticed until recently is because of "too few people looking into the biology and ecology of these organisms."

He says of the 42 locations in the world for which they have long-term data, coastal Japan is the only place to experience an unprecedented rise in jellyfish numbers. In this case, the culprit is the giant Nomura jellyfish (Nemopilema nomurai).

"These things have the weight of a sumo wrestler, are very difficult to study and create a lot of problems by breaking the nets of fisherman in Japan. It really doesn't go unnoticed … [so] we have records of when this organism has bloomed in the past for the last 120 years."

Environmental impact

Past research has linked increases in jellyfish numbers to overfishing - jellyfish eat zooplankton as well as fish eggs and larvae - and other environmental factors such as pollution and acidification.

"There's lots of mechanisms by which jellyfish populations could benefit from the degradation of the ocean, but there's not a lot of data out there to demonstrate that that's happening," says Dr Kylie Pitt, senior lecturer at Griffith University, another member of the JEDI research group.

She says long-term data is crucial to understanding what might be driving jellyfish boom and bust cycles.

"You need to look back not just over the last 10 years but potentially the last 50 years," says Pitt.

"Not all jellyfish populations will behave identically - even though on average, consistent patterns exist from across the world. Examining this [data] will help us to try and identify what environmental factors may be driving the fluctuations in jelly blooms."

This in turn will help scientists to target resources, she says.

"We really need to know what the important things are that we should be targeting. If you're directing a whole lot of resources towards something that doesn't exist then that's a waste of resources.